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Abelian Groups - László Fuchs [Springer]

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5 The Functor Pext 277<br />

Thus Pext.C; A/ is not only a subset, it is also a subgroup of Ext.C; A/, called the<br />

group of pure-extensions.<br />

If e W 0 ! A ˛!B ˇ!C ! 0 is a pure-extension of A by C, andif W<br />

A ! A 0 ;W C 0 ! C are homomorphisms, then the extensions e and e are<br />

again pure-extensions, this is easy to check directly, but also follows immediately<br />

from Theorem 5.2 by observing that the map Ext.; / W Ext.C; A/ ! Ext.C 0 ; A 0 /<br />

carries Ulm subgroups into Ulm subgroups. We can thus claim that Pext is an<br />

additive bifunctor Ab Ab ! Ab, contravariant in the first, and covariant in<br />

the second variable.<br />

From Theorem 4.3 in Chapter 5 we derive at once:<br />

Corollary 5.3. A group C satisfies Pext.C; A/ D 0 for all groups A if and only if it<br />

is †-cyclic.<br />

ut<br />

Exact Sequences for Pext Having had a first glimpse of Pext, it looks interesting<br />

enough to pursue its properties. To reveal its behavior towards exact sequences,<br />

we prove a couple of results. The next two lemmas are more general than we need<br />

them right now, but we will find their generality useful later on.<br />

Lemma 5.4. Suppose 0 ! A ˛!B ˇ!C ! 0 is an exact sequence such that ˛A is<br />

contained in the th Ulm subgroup B of B. Then ˇ maps B onto C .<br />

Similarly, if ˛A p B, then ˇ.p B/ D p C:<br />

Proof. It is pretty clear that ˇ.B / C , so only the surjectivity requires a proof.<br />

Since Ker ˇ B , the transfinite heights of the elements of B not in B are the<br />

same in every coset mod Ker ˇ. As the height of an element in an epic image is<br />

the supremum of the heights of its preimages, no element can belong to C without<br />

being contained in ˇ.B /.<br />

The proof for p B is the same.<br />

ut<br />

Lemma 5.5. If e W 0 ! A ˛!B ˇ!C ! 0 represents an element in the th<br />

Ulm subgroup Ext.C; A/ of Ext.C; A/, then for every group G, the connecting<br />

homomorphisms ı and ı act as<br />

ı W Hom.A; G/ ! Ext.C; G/ ; ı W Hom.G; C/ ! Ext.G; A/ :<br />

In particular, if e is pure-exact, then Im ı and Im ı are contained in Pext.C; G/<br />

and Pext.G; A/, respectively.<br />

Similarly, if e 2 p Ext.C; A/, then Im ı 2 p Ext.C; G/ and Im ı 2<br />

p Ext.G; A/.<br />

Proof. An 2 Hom.A; G/ induces a map W Ext.C; A/ ! Ext.C; G/ that carries<br />

the top row into the bottom row in the commutative diagram

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